Interleukin-7 (IL-7) is a cytokine with broad functionality in the immune system: it is critical for lymphocyte development and peripheral T cell survival. Recent evidence however indicates that polymorphisms in IL-7R? are associated with increased risk for various autoimmune diseases and blocking IL-7/IL-7R? interaction in mouse models of Type 1 Diabetes (T1D), Multiple Sclerosis, and Rheumatoid Arthritis has therapeutic benefit. Together, these studies strongly suggest that the IL-7/IL-7R? pathway plays a critical role in the development of autoimmunity. We found that IL-7R? blockade prevents and reverses autoimmune diabetes in non-obese diabetic (NOD) mice, which spontaneously develop a disease similar to human T1D. The main goal of this proposal is to understand the mechanisms underlying the role of IL-7/IL-7R? in the pathogenesis of T1D as a basis for developing novel immunotherapies targeting the IL-7 pathway. The central hypothesis is that IL-7 promotes T1D by enabling autoreactive CD4+ T cells to escape critical regulatory mechanisms, resulting in compromised tissue tolerance. Increased access to IL-7 due to production of the cytokine in the pancreas and altered IL-7 signaling in autoreactive T cells are potential mechanisms underlying IL-7-driven autoimmunity. Therefore, we will perform an in-depth analysis in NOD mice of IL-7 production at the autoimmune site, and of the role of IL-7 receptor signaling in effector/memory T cells and Tregs. In Specific Aim 1, we will investigate how IL-7 counteracts inhibitory receptor expression induced in autoreactive CD4+ T cells exposed to persistent islet self-antigens. The role of IL- 7R? expression levels and distinct IL-7 signaling pathways in preventing regulation of islet- specific T cells in the pancreas will be determined using novel phosphoflow cytometry. In Specific Aim 2, we will investigate the role of IL-7R? expression in pancreatic Tregs, using a novel conditional knockout mouse model allowing specific deletion of IL-7R? in Foxp3+ Tregs. Impact on T1D development will be evaluated. In Specific Aim 3, we will characterize IL-7- producing cells in the pancreas during progressive autoimmune inflammation and we will identify the signals inducing pancreatic IL-7 expression, using novel IL-7/GFP knock-in reporter mice. We have generated innovative mouse models to investigate how IL-7 drives autoimmune disease. Successful execution of the proposed experiments will provide critical knowledge to design novel strategies for therapeutic targeting of the IL-7/IL-7R? pathway in T1D and, perhaps, other autoimmune conditions.